Step 1: My Hold!
I have uploaded the .stl file to the instructable but I will also include a like to the page in thingiverse if the download isn't working for some reason. If you want to alter the design I'm happy to oblige, message me and I will email the .skp file! I can also provide a .bfb file if you are using a bits from bytes printer.
The hold was printed in PLA yellow and can hold my entire body weight (~175 lbs) when gripped with two hands on a ~60 degree angled wall. The jug can easily be gripped with two hands and makes a great start hold for a route. The printer I used left kind of a rough texture inside the jug but you can sand it to your delight. I do not recommend using ABS but if you try please post the results! To mount it to the wall I just used a 3/8'' carriage bolt with a washer and nut on the back side but you can just as easily use your standard hex bolt with a t-nut no problem-o.
NOTE: This hold needed a bit of drilling as the printer I used (Bits From Bytes) filled in the bolt hole somewhat. When drilling out the hold be sure you pilot with several different sized bits, the plastic will bite very easily. Also due to the symmetric nature of the hold I have had almost no issues with spinning but with the same technique you can easily pop a set screw through the top of the hold (that's why I included it).
Step 2: Your Own Hold!
(Print it out!)
When designing your own hold look for aspects of the hold on your wall the you like. Where is the thumb catch? How thick is the lip/radius? Are you looking to make a pocket, jug, crimp, ect? The best thing about designing a rock climbing hold is you really can't be wrong... You can just be better or worse, and that comes with practice.
When placing a bolt hole in your design you will typically be designing to accept 3/8" bolts, but remember the plastic will shrink so make it just a tad larger. Also be sure to include an inset of the top of your bolt. Measure the diameter and height of the head of the bolt and make your inset just a bit wider and taller.
The purpose of this instructable is largely to get the idea out there to make climbing holds! I'd love to see what the community can make and try them out, so if you have a design please share it with the world! :)